This invention relates to a new process for applying, in a controlled manner, a silicon coating on the surface of an electrically conductive material. Such process is especially useful in coating the walls of fluidized bed reactors which are used in the dehydrogenation of polysilicon.
High purity monocrystalline silicon is in great demand as a semiconductor material. The purity of the silicon is critical as impurities, especially metal and hydrogen, can render the silicon unacceptable for some electronic uses.
Most of the world's supply of semiconductor grade silicon is produced from polycrystalline silicon, i.e., polysilicon, which in turn is produced from the thermal decomposition of a silicon source, e.g., silane, trichlorosilane and the like. The thermal decomposition can conveniently be carried out in a fluidized bed reactor into which is fed the silicon source material and a carrier or fluidizing gas. Such processes are exemplified in U.S. Pat. No. 4,784,052, U.S. Pat. No. 4,784,840, U.S. Pat. No. 4,868,013 and U.S. Pat. No. 4,883,687, which patents are all incorporated herein as if fully set forth. The polysilicon product from these processes is a free flowing powder comprised of essentially spherical polysilicon granules having a particle size range of from about 150 to about 3000 microns. Average particle size is from about 600 to about 1100 microns.
Despite the high purity of these granules, they often-times contain entrained hydrogen in unacceptable amounts. To reduce the hydrogen content the polysilicon can be treated at high temperature to cause a net diffusion of hydrogen out of the granules. It has been found that this dehydrogenation can be efficiently carried out in a fluidized bed reactor in which the polysilicon granules are heated to temperatures within the range of from about 900.degree. to about 1300.degree. C., with about 1150.degree. C. being the preferred highest temperature. The fluidizing gas is hydrogen or argon, with hydrogen being preferred.
While the use of fluidized bed reactors is highly efficient in effecting dehydrogenation, the considerable turbulence that occurs within the reactor is conducive to an increase in product contamination if special precautions are not taken. The major contamination source is the metal or material which is abraded from the reactor walls by the turbulently moving polysilicon granules. This contamination can be easily obviated if the reactor surfaces in contact with the abrading polysilicon granules are of a material which is not a contaminant to the polysilicon, e.g., silicon. Non-abrading materials such as quartz, silicon carbide, etc., can be used. However their fragile nature and/or unavailability in the sizes needed argues against their use. Providing reactor parts of pure silicon, especially reactor walls, is problematical from a cost and design standpoint. Therefore, it is preferred to manufacture the walls from a convenient material, say graphite, and to then give the walls a coat of silicon. The coat of silicon can be applied prior to running the dehydrogenation process by thermally decomposing a silicon source, e.g., silane, in the reactor to yield silicon which coats out onto the reactor walls. This coating is formed at a temperature of about 600 .degree. C. and prior to running the much higher temperature dehydrogenation process in the reactor. After the coat is obtained no further silicon source is fed to the reactor. One disadvantage to using such a coating is that the coating will suffer attrition, cracking and flaking due to the abrading turbulent motion of the polysilicon, to thermal shock and to reactor vibration. When these deleterious effects become severe, which they will over time, exposure of the underlying wall material results. To prevent this exposure from occurring, the reactor walls can be periodically recoated. The recoating can be effected by shutting down the reactor as a dehydrogenator and, instead, running it to decompose a silicon source feed to generate silicon for the recoating. This periodic recoating, while effective, is not a panacea as the shut-downs interfere with production and can result in reactor damage due to thermal cycling. Thus, what is needed is a process which continuously regenerates a silicon coat on the reactor walls of a dehydrogenator during and without interruption of the dehydrogenation process.